Faction doesn''t mind being killed.
I found that for some reason, the Xenos, though sworn enemies, had a base that showed as neutral to me. I docked with it, bribed the Xenos to be friendly, and took a few missions to take out Rogues and other criminal factions competing with the evil Xenos.
Here's the weird part: apparently on missions where they send backup ships, you can complete the mission, then kill any of the remaining support fighters and bag their loot. They don't fight back, and it doesn't seem to effect how the faction feels about you.
I'm not sure if that's the normal behavior, but it was pretty cool. I made a bit more money selling the friendies weapons on top of the loot from the bad guys.
By the way, you can tell they are mission-spawned units when you hail them and they do not respond. Sometimes you will encounter random patrols, etc. and firing on them does not have the same non-effect!
"If it bleeds, we can kill it."